1. Field of the Invention
The invention relates to a solenoid-operated control valve with reduced resonance instability and reduced hysteresis.
2. Background of the Invention
Pressure regulation in a hydromechanical valve system typically uses an electronically-controlled valve actuating solenoid, wherein regulated control pressure developed by the valve system is functionally related to current applied to a solenoid actuator in accordance with a calibrated transfer function. Valve systems of this kind are used, for example, in automatic transmissions for automotive vehicles. Control pressure developed by the transmission valve system actuates fluid pressure operated clutch and band servos in a gearing system. The servos are controlled in this fashion to establish and disestablish multiple torque flow paths from an engine to a driven shaft in an automotive powertrain. Calibrated ratio shifts in the gearing are achieved as transmission clutch and band servos are selectively activated. Under certain operating conditions, such systems may be characterized by dynamic instability related to interactions between transmission pressure regulating solenoids and other elements of the hydraulic system.
It is known design practice to introduce a dither frequency for solenoid valve current, causing the current to oscillate at a predetermined high frequency and at a predetermined low amplitude. The dither frequency is applied to the solenoid energizing current to produce a variable magnetic field related to energizing current with a precalibrated transfer function. One type of solenoid used in automotive vehicle powertrains has an output pressure that is inversely proportioned to current. Typically, the output pressure for such solenoids obtained when the energizing current increases is less at a particular current level than the output pressure obtained for the same current level when the current level decreases. Because of this difference (hysteresis effect), the output pressure is not precisely proportional to a given magnetizing current.
The presence of dither frequency imposed on the energizing current improves reliability of the regulating valve by reducing the possibility of valve sticking and by reducing adverse performance of the system valve due to residual magnetism and changes in system compliance related to temperature changes, valve mass, valve spring force, and mass of the regulated fluid itself. The dither frequency also reduces adverse effects of fluid viscosity changes and contamination.
Instability of the operational characteristics of a variable force solenoid can be caused also by internal wear, which can cause a shift in the transfer function relative to an initially calibrated transfer function. This may produce a lower pressure for a given transmission torque demand, which may lead to undesirable elongated ratio shifts in the transmission, friction element flare, and clutch wear. Further, variations in the transfer function may cause undesirable high pressures for a given solenoid current level, which in turn can cause a decrease in shift quality, evidenced by harsh shifts, clutch wear, and friction element tie-ups as one friction element is actuated or released out of synchronism with the application or release of a companion friction element.
It has been found that in control systems of this type, in which a dither frequency is used to reduce variations in transfer function, the electrical hydraulic and mechanical performance of the valve system may change, thereby causing a resonance instability, which can cause unwanted steady-state variations in output pressure.
System compliance may be a function of the amount of air entrained in the fluid within the pressure control system, as well as within mechanical accumulators in the valve system. Such variations in compliance can cause problems due to resonance with respect to the dither frequency. Resonance can result in large amplitude valve oscillation, which can cause metal contact at an end of travel of a valve within its sleeve, thereby causing asymmetrical fluid supply and fluid exhaust flow that shifts the average pressure output of the valve.